This invention relates to transmission of trellis coded modulation (TCM) of digital voice and data, and more particularly to TCM in combination with multiple phase-shift-keyed (MPSK) signaling, and with asymmetry (nonuniformity) in the phase shift of the MPSK signal set for reliable high quality communications in the presence of fading conditions, particularly when limitations of power and bandwidth are imposed.
There is a growing need for reliable transmission of high quality voice and digital data in satellite-based land mobile communication systems. These systems, which will be part of an emerging all-digital network, are both power and bandwidth limited. To satisfy the bandwidth limitation, one can employ bandwidth efficient modulation techniques such as those that have been developed over the recent past for terrestrial microwave communications systems. Examples of these are multiple phase-shift keyed (MPSK) signaling, quadrature amplitude modulation (QAM) and the various forms of continuous phase frequency modulation (CPM). When power is limited, forward error correction (FEC) coding is ordinarily used.
When limitation of power and bandwidth are both imposed simultaneously, as in the mobile satellite application, it is most often not possible to achieve the desired data rate of 4.8 or 9.6 kilobits per second (kbps) with either technique acting alone. Instead, what is required is the integration of bandwidth efficient modulation scheme with some form of FEC coding to exploit the best possible attributes of both.
In the past, coding and modulation were treated as separate operations with regard to overall system design. In particular, most earlier coded digital communication systems independently optimized: (a) conventional (block or convolutional) coding with maximized minimum Hamming distance, and (b) conventional modulation with maximally separated signals.
About a decade ago, using random coding bound arguments, it was shown that considerable performance improvement could be obtained by treating coding and modulation as a single entity. (J. L. Massey, "Coding and Modulation in Digital Communications," Proc. 1974 Int. Zurich Seminar on Digital Commun., Zurich, Switzerland, March 1974, pp. E2(1)-(4).) Many years later, this concept was formalized into a rigorous theory which showed that optimally designed rate n/(n+1) trellis codes suitably mapped (to maximize Euclidean distance) into conventional 2.sup.n+1 -point signal sets can provide significant coding gain without bandwidth expansion when compared with uncoded conventional 2.sup.n -point signal sets. (G. Ungerboeck, "Channel Coding with Multilevel/Phase Signals," IEEE Trans. on Inform. Theory, Vol. IT-28, No. 1, January 1982, pp. 55-67.) It is this work that has laid the foundation for the design and development of all power and bandwidth efficient digital modems found in practice today and those that are to come in the future.
The most common application of such trellis coded modulation (TCM) techniques is in the new generation of modems being developed for the telephone channel. Indeed, the present state-of-the-art is a rate 6/7, 8-state trellis coded 128-point QAM which is capable of transmitting 14.4 kbps over good quality (D1-conditioned or better) leased telephone lines. (J. Payton and S. Qureshi, "Trellis Encoding: What it is and How it Affects Data Transmission," Data Communications, May 1985, pp. 143-152.) Thus if it is practical to send 14.4 kbps over the telephone channel, transmitting 4.8 or 9.6 kbps information for a 5 kHz satellite channel (typical of present considerations) might appear to be simple.
Several reasons make this supposition untrue most of which relate to the additional sources of degradation present on the mobile satellite channel but absent on the telephone channel. First, Doppler frequency shifts due to mobile vehicle motion can be a serious source of performance degradation if not compensated. Second, the fact that the 5 kHz mobile channel is actually a slot in a frequency-division multiple access (FDMA) system, brings on the problem of interference due to energy spillover from adjacent channels. Third, the satellite channel is inherently a nonlinear one primarily due to the holding and positioning aid (HPA) in the transmitter. Thus, one must either employ constant envelope modulations and operate at full power or, if using non-constant envelope, but bandwidth efficient modulations such as QAM, then the HPA operating point must be backed off in power to produce an approximately linear channel.
The most serious source of impairment that does not exist on the telephone channel is the combination of multipath fading and shadowing, i.e., for reliable performance, the system must combat short fades and recover quickly from long fades. Fading, which for mobile satellite channels can be assumed to be modelled by a Rician distribution, not only introduces an error floor into the system but also makes the problem of carrier recovery more difficult. Depending on the ratio of direct and specular (coherent component) to diffuse (noncoherent component) signal power, one might even be required to employ differentially coherent or noncoherent detection techniques, thus sacrificing the power saving associated with coherent detection. Finally, even if the above sources of degradation were absent, the power limitation imposed by the mobile satellite channel would preclude transmission at the high data rates achievable on the telephone channel. Also, whatever technique is decided upon must be able to be implemented and installed in the vehicle with a minimum of cost and complexity, perhaps two orders of magnitude less than that associated with a telephone channel modem.